A Late Paleocene age for Greenland’s Hiawatha impact structure

The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet. To date the structure, we conducted 40Ar/39Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40Ar/39Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact’s exact paleoenvironmental and climatic significance awaits further investigation.


Supplementary text
Descriptions of grains from glaciofluvial sand sample HW21-2016 that were selected for 40 Ar/ 39 Ar analysis. As the grains were too small to be mounted in thin section prior to 40 Ar/ 39 Ar analysis, only a brief description of the grain exterior was possible. Images of all analyzed grains are given alongside the 40 Ar/ 39 Ar spectra in Fig. S3.
QL01 Chemical sedimentary grain with presumed quartz ooids -note pale, almost perfectly shaped, ellipsoidal to spherical bodies of silica with nuclei of ?quartz fragments.
QL03 Melt grain with imperfectly shaped microspherulites of zoned feldspar in a dark brown glassy matrix with organic carbon, and pale fragments of quartz and (presumably) partially melted feldspar QL04 Felsic melt grain with closely packed, yellowish microspherulites with concentric structure.
Perlitic fracturing seems to be present. QL10 Grey melt grain with scattered, pale yellow, poorly developed spherulites and mineral fragments of mainly quartz. QL11 Melt grain with a dark brown, aphanitic matrix, a few indistinct microspherulites, pale yellow patches of presumed partially dissolved feldspar and fragments of quartz.
QL12 Melt grain with numerous, closely packed microspherulites with radiating marginal parts in a greenish grey matrix, dark, irregular aphanitic patches and fragments of quartz and feldspar.
QL14 Melt grain with dark, ?organic carbon-rich glassy matrix, a few small oblong, pale spherulites, yellow patches of ?partially dissolved feldspar and fragments of quartz.
QL15 Heterogeneous pale grain with light grey presumed melt matrix and abundant clasts. QL16 Black, apparently microporphyritic melt grain with microlites of presumed pyroxene and ilmenite in a hemicrystalline feldspathic matrix with organic carbon and fragments of quartz.   ArcticDEM (78) overlain with modeled subglacial water accumulation. The latter field is calculated using the Shreve hydropotential (79) and 100 realizations of BedMachine v3 bed topography with an additional normally distributed elevation uncertainty whose standard deviation is 200 m. These water pathways are then summed into the total count shown. In this model, subglacial water is permitted to flow everywhere (a conservative scenario, given that the bed is likely frozen in some regions; (30)), but that water is only produced uniformly inside of the apparent crater rims. In this manner, the effect of subglacial and subaerial transport from the two structures can be isolated from other possible confounding factors. (B) BedMachine v3 bed topography (23). Assuming that samples HW19-01 and -05 were recently transported subglacially, prior to their proglacial discharge and subaerial recovery, this analysis demonstrates that they most likely originated from the Hiawatha impact structure, and the second possible impact structure ("Paterson"; (25)) is unlikely to have recently contributed significant material to the sampled floodplain. Any subglacially entrained samples from that second structure are consistently routed farther north, toward the floating terminus of Humboldt Glacier. Further, samples HW19-01 and -05 are found at the intersection of two of the primary modeled transport routes for material from beneath Hiawatha Glacier.                            (Fig. 4A, fig. S3). Note that the inverse isochron ages are identical to the mini-plateau ages and have 40 Ar/ 36 Ar intercepts that are within error of the atmospheric value. Int. -intercept; MSWD -mean square of weighted deviates.